Transformer-based power supplies often cycle between two or more phases when converting a first voltage to a second voltage. Between phases, the transformer will commutate (i.e., reverse the transformer voltage). To allow the transformer to commutate, the power supply must first disconnect the current path on the primary side of transformer. When the power supply has a large load current, however, energy stored in the transformer's leakage inductance may cause problems such as ringing, large voltage spikes, or fast commutations—all of which generate noise that can distort the transformer voltage.
For example, if commutation occurs too quickly, the sudden change of voltage across the transformer can cause an undesirable displacement current—called common mode (CM) current—to flow between the transformer's primary and secondary windings. The speed of the commutation depends on the magnitude of the transformer's magnetization current and the size of the commutation capacitance (Cc) that it must charge in order commutate the transformer. One method to slow down the commutation (and reduce the CM current) is to add one or more additional capacitors to the transformer. This effectively increases the commutation capacitance (Cc), which is the sum of the normal mode capacitance and any additional capacitors.
But this method does not work well for large load currents. During commutation, the energy stored in the transformer's leakage inductance forces a change of the voltage on Cc. This leakage inductance energy (Eleakage) is related to the load current by the equation Eleakage=½Lleakage Iload2, and increases with respect to the load current. By changing the voltage on Cc, Eleakage may increase the commutation rate (thereby increasing undesirable CM current) when the leakage energy is significant with respect to the commutation energy (Ec) (defined as Ec=½CcVc2, where Vc is the commutation voltage). A significant relationship typically exists when Eleakage≧Ec/2. As mentioned above, a larger Eleakage may also cause other undesirable effects, such as ringing or large voltage spikes.
One solution for managing larger Eleakage caused by high load currents has been to slow the rate at which switches in the power supply disconnect the primary side current. During the transition from on to off, the switch resistance will dissipate some of the leakage inductance energy as heat. A slower switch transition allows the switch to dissipate more leakage energy, and therefore less energy is transferred to the primary capacitance. For very high load currents, however, the switches often get too hot to use this approach.
Thus, there is a need for improved techniques to minimize the effect of large load currents on the transformer commutation.